I think the point behind the visible lifeboat hatches was to have justifiable detail on the hull. No one wants Star Wars-style nurnies all over a Starfleet ship, but there's an instinctive need in SFX people to give us little details for scale reference and to break up smooth surfaces. So in later Trek productions, there's a search for excuses for details ... escape pod hatches, phaser emitters, photon torpedo launchers. Sensor palettes, tractor emitters, transporter arrays -- until eventually we get ships like the 1701-E and 2009 Enterprise
that are practically covered with nurnies.